deal2lkit: A ToolKit library for Deal.II
deal2lkit documentation

# Introduction

deal2lkit is a collection of modules and classes for the general purpose finite element library deal.II. Its principal aim is to provide a high level interface, controlled via parameter files, for those steps that are common in all finite element programs: mesh generation, selection of the finite element type, application of boundary conditions and many others. Each module can be used as a building block independently on the others, and can be integrated in existing finite element codes based on deal.II, drastically reducing the size of programs, rendering their use automatically parametrised, and reducing the overall time-to-market of finite element programming. Moreover, deal2lkit features interfaces with the SUNDIALS library (SUite of Nonlinear and DIfferential/ALgebraic equation Solvers) and with the Assimp library (Open Asset Import Library).

The deal2lkit library is released under the GNU Lesser General Public License (LGPL) and can be retrieved from the deal2lkit repository https://github.com/mathLab/deal2lkit.

# Motivation

The solution of partial differential equations by means of a finite element method always requires at least the following steps:

• generation of a geometrical grid to represent the domain of the simulation;
• definition of the discrete functional space for the solution;
• application of proper boundary conditions;
• actual solution of the algebraic problem;
• post-processing of the result (data output and error analysis).

Such a structure usually implies that different problems share a considerable amount of code. A natural response to such common background lies in the use of open source libraries as building blocks for advanced numerical solvers. The general purpose finite element library deal.II is one of the most successful libraries of this kind, and allows considerable simplification when writing complex finite element codes.

The deal.II library has been written with generality in mind, and allows the solution of several classes of finite element problems. Its flexibility can be attributed to the granularity and modularity of the code base, in which only the building blocks of finite element codes are programmed, and the semantic for the solution of an actual problem is left to users of the library. This approach has the advantage that deal.II can be used to solve virtually any problem that can be written into a partial differential equation, but leaves to the user the burden to stich together the various building blocks. A typical approach is to start from one of the many example programs that the library comes with (more than 50), and modify it to suite the needs of the user. While the approach copy-modify-run may be well suited for a single person working on a single project, it falls short when one wishes to reuse the same code base to solve possibly very different problems. The biggest difficulty comes from the fact that most of the tasks above have slightly different specialisations depending on the problem at hand. These specialisation are usually difficult to generalize, since they depend, for example, on the number of variables of a problem, the types of boundary conditions one would like to impose, or the type of norm one would like to use when computing errors during the post-processing phase of a program.

deal2lkit is a library of modules built on top of deal.II that drastically reduces the amount of repeated lines of code between different projects, by introducing an extensive use of parameter files into every step of a general finite element code.

deal2lkit features also interfaces for other scientific libraries in order to tackle problems of increasing difficulties. So far we have constructed convenience wrappers around the following external libraries:

deal2lkit is distributed under the free GNU Lesser General Public License (LGPL) and is available from the deal2lkit repository at https://github.com/mathLab/deal2lkit. The library is tested by means of the continuous integration service hosted by Travis CI (https://travis-ci.org/).

# Modules overview

## ParameterAcceptor: the base of all deal2lkit classes

In general, a parameter file is used to steer the execution of a program at run time, without the need to recompile the executable, with clear advantages in terms of human-time.

In the deal.II library, reading and writing parameter files is done through the ParameterHandler class, that provides a standard interface to an input file that can be used to feed run-time parameters to a program, such as time step sizes, geometries, right hand sides, etc.

deal.II supports the standard xml or JSON formats, or a custom text format which resemble bash files with support for sections, as in the following example:

subsection Nonlinear solver
# this is a comment
subsection Linear solver
set Solver = CG
set Maximum number of iterations = 30
end
end

Typically, the following four steps are required to let a program use a parameter file:

• make sure that the program knows what entries will be in the file;
• create a parameter file with default values if one does not exist;
• parse all entries of the file (possibly raising exceptions if the entries were not previously declared, or if the parsed entries contain illegal values);
• assign the parsed entries to local variables of the program.

The ParameterHandler class of the deal.II library provides facilities to perform the above four steps, through the following methods:

In large programs, where the number of parameters easily exceeds hundreds of entries, managing the above four actions for different classes is far from trivial. The deal.II documentation advocates the creation of a class that would store all parameters of the problem, with two methods:

• declare_parameters(prm)
• parse_parameters(prm) or get_parameters(prm)

that should be called by the program before writing or reading a parameter file, and right after having read the parameter file into an object prm of type ParameterHandler

Such an approach has the advantage that bookkeeping is simple, if compared to a scattered approach where each class keeps track of its own parameters, but it suffers one big draw back: it is not reusable for problems of different type and it has still the defect that one has to separate declaration and recovery of each parameter, as in the following short example:

void NonLinEq::declare_parameters (ParameterHandler &prm) {
prm.enter_subsection ("Nonlinear solver");
{
prm.declare_entry ("Nonlinear method",
"Newton-Raphson",
ParameterHandler::RegularExpressions::Anything());
eq.declare_parameters (prm);
}
}

The complementary part of this code is contained in the parse_parameters method, which actually fills the values of the local variables.

void NonLinEq::parse_parameters (ParameterHandler &prm) {
prm.enter_subsection ("Nonlinear solver");
std::string method = prm.get ("Nonlinear method");
eq.parse_parameters (prm);
}

According to the proposed design in the deal.II documentation, such separation is necessary (with a consequent proliferation of several places where one has to keep track of what variables have been declared and what variables have been assigned locally) since the declaration, reading and writing of a parameter file, and the assignment to local variables have to be done exactly in this sequence.

deal2lkit implements a global subscription mechanism and a local subscription mechanism through the base class ParameterAcceptor, which maintains compatibility with all classes written following the deal.II suggested construction, and provides an additional method which removes the necessity to split the declaration and parsing of parameters.

The global subscription mechanism is such that whenever a class that was derived by ParameterAcceptor is constructed, a static registry in the base class is updated with a pointer to the derived class. Such registry is traversed upon invocation of the single function ParameterAcceptor::initialize(file.prm) which in turn calls the method ParameterAcceptor::declare_parameters() for each of the registered classes, reads the file file.prm, (creating it first with default values if it does not exist) and subsequently calls the method ParameterAcceptor::parse_parameters(), again for each of the registered classes.

## Preprocessing

Of the basic steps for any finite element code, the intial preprocessing phase (mesh generation, mesh import, definition of finite element spaces, definition of the quadrature formulas to use, definition of the boundary conditions and of the forcing terms, etc) are among those tasks which are almost equal in every user code.

deal2lkit provides the following preprocessing classes

that help in the definition of a finite element program, by creating parsed interfaces for all common tasks for the precprocessing phase of any finite element code.

## Postprocessing

When a solution is available, usual postprocessing tasks for finite element codes include the computation of errors w.r.t. exact, known, solutions, and the output of the computed solution.

Also in this framework, deal2lkit offers some classes to simplify through parameter files the above tasks: